In this article we will discuss about the cultural, physical, biological and bioherbicidal control of weeds.
Any weed management programme on farm land essentially begins with the adoption of good crop husbandry practices leading to a sturdy crop which could overpower the weeds, and make their subsequent control easy and more economical. On the contrary, when the fundamental principles of good crop husbandry are ignored, the crops become so weedy that their control is often disappointing.
Some important good crop husbandry practices which can weaken and bring about effective suppression of weeds in farming systems are as follows:
(i) Proper Crop Stand:
Gapy and under population crops are prone to heavy weed infestations which become difficult to control later. Therefore, practices like selection of proper seed, right method of sowing, use of adequate seed rate, and protection of seed from soil borne pests and diseases are very important to obtain proper and uniform crop stand capable of offering significant competition to the young weeds.
In this respect it is also essential to adopt a sowing method which will ensure emergence of the crop prior to the weeds. As a matter of ecological principle, the first plants to appear on the ground tend to eliminate the later visitors.
(ii) Selective Crop Stimulation:
Basal placement of fertilizers in the seed rows often helps in selectively stimulating the crop seedlings which can withstand competition from the weeds much better. The initially healthy crop seedlings also grow fast and close-in early to shade out the subsequent flush of weeds.
In this respect the various plant protection measures adopted to maintain the crop plants healthy should be considered as part of good crop husbandry, leading to effective weed management. The top dressing of a fertilizer in the inter-rows should be done in a manner that major portion of it is utilized by crop plants, and the weeds are able to make only limited use of it.
If the soil was of abnormal pH, not suited to the healthy growth of the crop intended to be sown, the use of suitable soil amendments should be considered necessary to boost crop growth and, thus, indirectly suppress the weeds.
(iii) Crop Rotation:
Many of our weed problems that exist with us today are largely because of practising monocultures i.e. growing of the same crop year after in the same field. For instance, growing of rice after rice throughout the year in the southern and eastern states of India has led to several difficult weed problems. In some places, introduction of banana as a break crop has solved this problem.
Other break crops must be found for each local situation if the weeds in rice fields of these areas are to be brought under check in a feasible manner. Similarly, in the wheat belt of north India, monocultures have led to some challenging weed problems which are more difficult to control than the previous ones. Proper crop rotations must be followed in these areas to give a break to the problem weeds.
Parasitic weeds, as well as the crop associated weeds, can be likewise discouraged by adopting well-conceived crop rotations. A crop like potato, which demands considerable soil disturbance during its earthing, is a good crop to include in rotation to dislodge the weeds.
Similarly, grassy weeds like Phalaris minor and Avena fafua can be easily discouraged by replacing wheat with a frequently grazed or cut forage crop in alternate years. Our indifference to long term farm plans involving well-conceived crop rotations is a big reason for our ever growing weed problems.
In essence, the break crops in crop rotations discourage weeds by- (i) providing a widely differing microclimate to the weeds under its canopy; (ii) refusing its parasitism by the parasitic weeds; (iii) offering intensive soil upheaving in certain cases, (iv) giving opportunity for intensive inter-row cultivation if it was a wide row crop; and frequent grazing or cutting when a suitable forage crop was selected.
(iv) Summer Tillage:
The practice of summer tillage adopted by our forefathers has declined in the recent past. This has so happened in response to the increased cropping intensities adopted by the farmers after the expansion of irrigation facilities in the country. This is again undesirable from the point of view of control of weeds, particularly of the perennial ones.
In major part of India there is a distinct solar energy-rich, dry summer period available, which should be utilized for the desiccation of rhizomes, tubers, and roots of the perennial weeds, leading to loss of their vigour and even complete kill. Initial tillage of the field in summer for this purpose should encourage clod formation. These clods upon drying desiccate the weed propagules within much better than a pulverised soil.
The subsequent tillage operations should break these clods into smaller units to further expose the shrivelled weed propagules directly to the hot sun. Scientific studies have been conducted with certain perennial weeds to find their optimum growth stages for practising summer tillage when the downward translocation of food from their leaves to the rhizomes shall be minimal.
Most studies indicate that an interval of 15-20 days after their fresh shoot emergence was optimum for the purpose. It is expected that eventually after a few years of properly spaced tillage programme the underground parts of the weeds can be weakened to acceptable levels. Dry weather and dry soil are two important pre-requisites for the success of summer tillage as a weed management component.
(v) Reduction in Area under Bunds (Levees):
It is a matter of common observation that weeds grow profusely on bunds and channels of crop fields. Also, these weeds are often ignored by the farmers since they don’t compete directly with crop plants. In the western countries, to obviate this, plastic sheets are used for bifurcating the land into small fields for the purpose of irrigation.
In India, where plastic sheets are yet costly, reduction in the frequency of soil bunds by proper levelling and grading of the fields should be given due consideration in any good weed management plan.
(vi) Stale Seedbed:
A stale seedbed is one where the initial 1-2 flushes of the weeds are destroyed before planting of a crop. This is achieved by soaking a well prepared field with either irrigation (or rain) and allowing the weeds to germinate. At this stage a shallow tillage or a non-residual herbicide like paraquat may be used to destroy the dense flush of young weed seedlings.
This may be followed immediately by sowing a desired crop. The technique allows the crop to germinate in almost a weedfree environment. Spike-tooth harrow is a very useful implement for destroying the emerging weeds during the preparation of stale seedbeds.
This is another method of utilisation of solar energy for the desiccation of weeds. In this method the top 5 cm soil temperature is further raised by 5-10°C by covering a pre-soaked fallow field with thin, transparent plastic sheet. The plastic sheet checks the long wave back radiation from the soil and also prevents loss of energy by hindering moisture evaporation.
In some regions the technique raised the soil temperature to as high as 56°C, which was sufficient to kill several kinds of weed seeds. In many experiments, eight weeks or longer treatment reduced the weed seed viability by 80% or more. Israel has so far made maximum use of this technique as a part of integrated weed management. It may be revealed here that soil solarisation is not much effective against the vegetative propagules and so the perennial weeds may sometimes overtake the annuals.
Trash mulching, although an old practice, has never yielded satisfactory results in terms of withholding the weeds. Most weeds, particularly the sedges and grasses penetrate easily through it. Paper mulches have also failed in this respect. Of recent, black plastic mulches of a about 1000 gauge thickness have proved useful in suppressing weeds and conserving soil moisture at the same time.
Presently these mulches are used only in advanced countries in certain high value, wide row vegetable crops and ornamentals which are planted or transplanted through the holes made at the desired spacings in the plastic strips. The plastic mulches go very well with the drip irrigation system. These mulches have also been found very useful for covering rings of newly planted orchard and forest trees.
In limited rainfall areas these rings are so mended as to slope towards the tree bases for making best use of the rain water. The plastic mulched rings have been found in experiments conducted in India to boost the growth of trees in comparison to the other mulching materials. The use of plastics in agriculture for different purposes, including for mulching, has been named plasticulture. It is slowly gaining ground in Indian agriculture too.
Many a time intercropping helps in the suppression of at least the secondary growth of weeds that occurs after the intercrop has fully covered the ground. It means that the initial weed growth in an intercropping system should either be prevented by some good crop husbandry method or controlled later with either a suitable soil-active herbicide or by hand-weeding.
The weeds occurring before the intercrop has shaded the inter-row spaces can be very troublesome since mechanical weeding cannot be adopted in this cropping system. Cowpea, soybean, groundnut, redgram, blackgram, and greengram are common intercrops in India used to increase the total crop productivity and suppress the secondary growth of weeds. The results vary.
A fast shading intercrop is also known as smother crop e.g. cowpea and soybean.
It is now well accepted that the primary objective of any tillage programme on the farm is to destroy weeds. If the weeds were effectively controlled by some other method, like by the use of herbicides, the tillage frequency could be reduced sometimes to zero level, in the production of crops.
In India, however, 2-4 preplant tillage operations are still the most practical means to obtain initially weedfree seedbeds for sowing of the crops. An ideal physical soil condition for sowing of the wide row crops is to obtain a firm tilth in the seed rows (=seedbed) and a loose tilth in the inter-rows (root bed). This will ensure rapid germination of the crop seeds but slow weed emergence in the inter-row spaces.
Inter-row tillage, also called intercultivation, is widely conducted to remove the weeds largely from between the crop rows, leaving the intra-row weeds largely unhurt. To some extent this situation can be improved if the inter cultivation is practised when the weeds are in their seedling stage. At this stage the stirring of the soil close to the crop rows, which occurs during inter-cultivation, will often disturb the intra-row weeds sufficiently to dislodge them.
There are two serious limitations with intercultivation as a physical method of weed control- First; it is not workable when the field is either too dry or too wet. Often, in the rainy season the farmers in the heavy soil areas have to wait for days together before the soil attains the right physical condition for intercultivation. During all this period the weeds grow fast and over power the crop. Second, the inter-row cultivations cut the surface roots of crops during the operation, which may give a setback to the crop.
The success of an intercultivation operation depends upon the kind of implement used and the stage of weed growth when it is operated. For example- in the slow germinating crops like potato and sugarcane, blind hoeing with either handblades or bullock and tractor-drawn spiketooth harrows will dislodge most of the young and germinating weeds. The spiketooth harrow can also be used in crops like maize and millets after their germination at their 2.5-5.0 cm height stage.
At this stage the implement can be run across the field without fear of breaking the crop seedlings. At a later stage, the second flush of weeds can be easily damaged with sweeps and other kinds of cultivators. Thus, the physical control of weeds requires planning from the beginning of the crop season.
Usually 2-3 kinds of implements are needed to achieve the desired results, depending upon the crop stage and the soil condition. Sweeps and blade harrows cut the weeds below the ground and leave the foliage flat on the soil surface to act as mulch. This may be particularly useful in the dry farming areas.
In the narrow row crops, hand-weeding is the only physical method available for removing weeds. Although very effective, under the present enormous like in labour wages it is often a very costly operation, unless the field area was small enough to be covered by farmer’s own family in spare time.
On light soils hand hoes of various shapes and sized often prove more efficient and economical than the hand blades. On heavier soils, on the other hand, Kudalis are more effective. For weeding rice fields in standing water, several designs of rotary weeders are available. To use these effectively the rice seedlings must be transplanted in 25 cm apart or wider rows.
For a more detailed note on the physical weed control tools and machines, the readers may refer to Hosmani (1991).
Method # 3. Biological Control:
Biological control of weeds involves the use of living organisms against them. These living organisms, called bioagents, could be insects, plant pathogens, herbivorous fish, snails, or even competitive plants. The philosophy behind this classical biological weed control is that in nature the growth of many a weed remains limited due to biotic stresses from their natural enemies, largely, insect pests and plant pathogens.
But when such a weed disperses from its native place to an alien area, it leaves its natural pests back home, and thus gets opportunity to spread uninterrupted in its new environment by leaps and bounds. The biologists then tend to search natural enemies of such weeds in places of their origin and import these to build up a strong biostress on the weed in question.
The introduced bioagents are tested thoroughly for their refusal to attack any plant of economic importance in the new environment before it is released for controlling the weed. The success of an introduced bioagent depends upon its adaptation to the new environment in terms of its survival, rapidity of its multiplication, and rapidity with which it can damage the weed.
Once a tested bioagent is released on a large infestation of the target weed, slowly the weed population declines/while that of the bioagent rises to a peak. At this stage the food for the bioagent falls short to maintain its large population. The result is a decline in the bioagent population whence the weed infestation again tends to build up.
Such bioagent and weed cycles continue to recur till a balance is reached between the two and the weed infestion stabilises at a very low level. At this stage the land can either be cropped or the native vegetation be allowed to take over from the weed.
The first successful attempt to employ an insect bioagent was begun in 1902 against the brush weed, Lantana camara, in Hawaii. A team of entomologists visited Central America and Mexico; the native land of the weed, and succeeded in finding Crocidosema lantana Busk, a moth, as the most promising bioagent of this semi- woody, perennial weed.
Since then certain other insects have also been found effective against this bush in different countries. Unfortunately, none of these insect bioagents has been established in India on lantana thus far. Nevertheless, three other noxious weed species have been very successfully controlled in the country certain the insect bioagents.
These weeds are- Opuntia dillenii, Salvinia molesta and Parthenium hysterophorus. Thousands of hectares of land under O. dillenii in Tamil Nadu and Maharashtra were recovered from the weed by the release of cochneal scale insect Dactylopius tomentosus Auct. The insect was imported from Mexico. The project was so successful that now it is difficult to find a patch of this weed there.
In Kerala, waterbodies and paddy fields have been freed of the noxious fern, S. molesta, with a beetle called Cytrobagous salviniae Sands. The insect is native of South America. It is released by scattering some pre-infested plants of salvinia over the weedy waters. It takes about 4-6 months for the salvinia mat to go yellow, and another six months to sink completely.
The young larvae of the bioagent damage the terminal buds, rhizomes, and petioles of the floating weed. By now the control of salvinia, which had made life of 5 million people miserable in Kerala, is so perfect that it is no more considered a noxious weed there. However, the ecological shift in weed flora may create new aquatic weed problems in the near future which should be properly monitored.
The latest, highly successful example of biological control of weeds in India is that of carrotgrass (Parthenium hysterophorus) which has occupied several thousands of land throughout the country. The most efficient bioagent against this noxious weed released in the country is a leaf eating Mexican beetle, Zygogramma bicolorata.
Waterhyacinth (Eichhornia crassipes) has been very successfully controlled in Florida (USA) with the hycinthmoth, Sameodes albigultalis B. Benner, which is native to South America. The bioagent larvae feed rapidly on the young leaves and buds of hyacinth. Besides it, two bettles, Neochetina eichhorniae Warner and N. bruchii Austache also damage the hyacinth foliage but these do not reproduce as fast as the hyacinth moth.
Besides the insect bioagents, in several countries of the world, a herbivorous fish called grasscarp or white amur (Ctenopharyngodon idella) has been employed as bioagent to control aquatic weeds. The carp is native to cold, fresh waters of China where it breeds freely but shows poor appetite for the aquatic weeds.
However, when it is transported to warm waterbodies of the tropics and subtropics, it turns to be a very shy breeder but develops a very strong appetite for several kinds of submersed weeds. In warm waters the carp can consume each day aquatic weeds equal to its own body weight. A fully grown carp is as heavy as 32 kg and it measures up to 1 m in length.
An important feature of the classical biological weed control described thus far, is that the whole operation is handled by the scientists and extension officers as one time operation of release of the bioagent and monitoring it.
During the last one decade a new approach to biological weed control, called bioherbicidal control, has been developed which is completely private companies oriented. This bioherbicide approach involves the spray of specific fungal and microbial spores (or their fermentation products), on the target weed. These preparations, called mycoherbicides, then release their phytotoxins to destroy the menace.
The concerned firms Sell these made to order products to the farmers as sprayable formulations, just like herbicides. Although none of these preparations has been tried in India, yet it may be useful to know about some successful bioherbicidal products in use abroad.
A mycoherbicide can be both, a specific and non-specific bioagent. Each regrowth of the weed must be treated with it just as in the case of using contact herbicides.